The present invention relates to an improved cyclic process for preparing a hydroxylammonium salt solution and converting a ketone to an oxime with the hydroxyl amine or hydroxylammonium salt. The invention further relates in particular to an improved process for the preparation of cyclohexanone oxime from cyclohexanone.
The cyclic process for the preparation of oximes from hydroxylamine or a hydroxylammonium salt is carried out in an acidic, buffered, aqueous reaction medium containing buffer acids such as phosphoric acid, bisulfate or buffer salts derived from these acid and mixtures thereof. The reaction medium is circulated between a hydroxylammonium salt synthesis zone, where nitrate ions, which have been added to the reaction medium, are catalytically reduced with molecular H.sub.2 to hydroxylamine, and an oximation zone where a ketone is added to react with the hydroxylammonium salt to produce an oxime. The nitrate ions consumed in the hydroxylammonium synthesis zone are added to the circulating reaction medium just before the reaction medium is introduced into the hydroxylammonium salt synthesis zone. The nitrate ions are generally added in the form of nitric acid of approximately 60 weight percent.
The nitrate ions in the hydroxylamine synthesis zone are first converted into hydroxylamine which in turn reacts with the available buffer acid in the reaction medium, forming the corresponding hydroxylammonium salt. The resulting solution obtained, containing hydroxylammonium salt, is withdrawn from the hydroxylamine synthesis zone and circulated to the oximation zone, where the hydroxylammonium salt, together with a ketone, which is also fed to the oximation zone, forms the corresponding oxime, with liberation of acid. The oxime is removed from the oxime synthesis zone. The reaction medium withdrawn from the oxidation zone contains small amounts of oxime and ketone. This aqueous reaction medium is then returned to the hydroxylamine synthesis zone.
The chemical reactions taking place during the successive steps of the process for the preparation of cyclohexanone oxime wherein the reaction medium comprises a solution containing phosphoric acid, are as follows:
1. formation of hydroxylammonium salt in the hydroxylammonium salt synthesis zone: EQU 2 H.sub.3 PO.sub.4 + NO.sub.3 .sup.- + 3 H.sub.2 .fwdarw. NH.sub.3 OH.sup.++ 2 H.sub.2 PO.sub.4 .sup.- + 2 H.sub.2 O
2. formation of cyclohexanone oxime in the oximation zone: ##SPC1##
3. make-up, in the form of HNO.sub.3, of nitrate ions consumed, after the oxime formed has been separated from the reaction mixture: EQU H.sub.3 PO.sub.4 + H.sub.2 PO.sub.4 .sup.- + HNO.sub.3 .fwdarw. 2 H.sub.3 PO.sub.4 + NO.sub.3.sup.-
following the make-up of HNO.sub.3, a solution is again available which, after removal of both the water formed by the reaction and the water introduced with the nitric acid make-up, will, theoretically, have the same composition as the initial solution used for the formation of hydroxylammonium salt. This solution is then circulated back to the hydroxylamine synthesis zone. The reduction of the nitrate ions in the hydroxylamine synthesis zone is accomplished in the presence of a catalyst; usually a palladium catalyst is used. The palladium is suspended on a carrier material of carbon or aluminum oxide. The carrier material usually has suspended thereon, for instance, 5-20 weight percent of palladium.
Organic substances, such as the ketone which is to be converted into oxime, and the resulting oxime itself, have an adverse effect on the activity of the catalyst if allowed to come into contact with the catalyst. To prevent the catalyst from being thus poisoned, the circulating reaction medium must be purged of the ketone and oxime contaminants prior to its entering into the hydroxylamine synthesis zone. The ketone and oxime content of the reaction medium should preferably be reduced to a value of not more than 0.02 percent by weight before the reaction medium is recirculated to the hydroxylamine synthesis zone.
In the processes disclosed in the prior art, the aqueous, weak acid reaction liquid coming from the cyclohexanone oxime synthesis zone is treated by means of a stripping process in an attempt to reduce the amount of organic compounds present.
Unfortunately, however, commercial operation of this process including purification of the reaction medium coming from the oxime synthesis zone by stripping, continues to be plaqued by residues of ketone or oxime in the aqueous reaction medium which are recycled to the hydroxylammonium synthesis zone. The processes are thus still severely hampered by poisoning of the catalyst.
In the stripping process the ketone is distilled off and the oxime is hydrolyzed according to the reaction ##SPC2##
Depending on the temperature, an equilibrium state slowly develops in the liquid being stripped with an equilibrium amount of oxime residue left, which is carried along with the bottom product withdrawn from the stripping column.
It has also been proposed to convert the organic residues into harmless products by heating the circulating reaction medium after the make-up by nitric acid for a time and at such a temperature that the harmful residues are decomposed or oxidized into compounds which are harmless to the activity of the catalyst. The time required to convert the residues to harmless compounds depends in particular on the degree of acidity of the solution and on the temperature; in general, a reaction time of about one hour has been found to be necessary. To maintain the extended reaction time required, the reaction solution circulating from the oxime synthesis zone to the hydroxylamine synthesis zone must be held up in a sizeable reservoir, requiring in addition to the large equipment an equally large volume of reaction solution.
To avoid poisoning the catalyst, the oxime residue must be removed from the circulation liquid or rendered harmless, for instance by an oxidation or decomposition process which converts the oximes to other compounds which do not poison the catalyst.